8 August 2017 (JRC) – Heatwaves amplified by high humidity can reach above 40°C and may occur as often as every two years, leading to serious risks for human health. If global temperatures rise with 4°C, a new super heatwave of 55°C can hit regularly many parts of the world, including Europe.

A recently published study by the Joint Research Centre (JRC) – the European Commission's science and knowledge service – analyses the interaction between humidity and heat. The novelty of this study is that it looks not only at temperature but also relative humidity to estimate the magnitude and impact of heat waves.

It finds out that the combinations of the two, and the resulting heatwaves, leave ever more people exposed to significant health risks, especially in East Asia and America's East Coast.

Warm air combined with high humidity can be very dangerous as it prevents the human body from cooling down through sweating, leading to hyperthermia. As a result, if global warming trends continue, many more people are expected to suffer sun strokes, especially in densely populated areas of India, China, and the US.

The study analyses changes in yearly probability for a high humidity heatwaves since 1979 under different global warming scenarios. If global temperatures increase up to 2 C above pre-industrial levels the combined effect of heat and humidity (known as apparent temperature or Heat Index) will likely exceed 40°C every year in many parts of Asia, Australia, Northern Africa, South and North America. Europe will be least affected with up to 30% chance of having such strong heat wave annually.

However, if temperatures rise to 4°C a severe scenario is on the horizon. Scientists predict that a new super-heatwave will appear with apparent temperature peaking at above 55°C– a level critical for human survival. It will affect densely populated areas such as USA's East coast, coastal China, large parts of India and South America. Under this global warming scenario Europe is likely to suffer annual heatwaves with apparent temperature of above 40°C regularly while some regions of Eastern Europe may be hit by heatwaves of above 55°C.

The authors highlight that although some urban areas such as Chicago and Shanghai are not considered to have high risk for heatwaves based on temperature only, the probability of extreme weather strongly increased when considering relative humidity.

According to the study, the effect of relative humidity on heatwaves' magnitude and peak might be underestimated in current research. The results of the study support the need for urgent mitigation and adaptation action to address the impacts of heatwaves, and indicate regions where new adaptation measures might be necessary to cope with heat stress.

ABSTRACT: The co-occurrence of consecutive hot and humid days during a heat wave can strongly affect human health. Here, we quantify humid heat wave hazard in the recent past and at different levels of global warming. We find that the magnitude and apparent temperature peak of heat waves, such as the ones observed in Chicago in 1995 and China in 2003, have been strongly amplified by humidity. Climate model projections suggest that the percentage of area where heat wave magnitude and peak are amplified by humidity increases with increasing warming levels. Considering the effect of humidity at 1.5° and 2° global warming, highly populated regions, such as the Eastern US and China, could experience heat waves with magnitude greater than the one in Russia in 2010 (the most severe of the present era). The apparent temperature peak during such humid-heat waves can be greater than 55 °C. According to the US Weather Service, at this temperature humans are very likely to suffer from heat strokes. Humid-heat waves with these conditions were never exceeded in the present climate, but are expected to occur every other year at 4° global warming. This calls for respective adaptation measures in some key regions of the world along with international climate change mitigation efforts.

[…] The occurrence of heat waves with AT55C, never recorded in our data records in the recent past, is likely to cause heat strokes by limiting the human thermoregulation. The exceedance of this apparent temperature across these regions is in agreement with other measures accounting for the combined effect of temperature and relative humidity. As an example, the wet-bulb temperature peak during a heat wave is expected to exceed the value of 35 °C (see Supplementary Fig. S9), a threshold likely to induce hyperthermia in humans and other mammals as dissipation of metabolic heat becomes impossible25, 31 While this never happens in the present climate, and it is unlikely at 1.5 °C and 2 °C, it would occur on a regular basis in many highly populated regions with global-mean warming of about 4 °C, questioning the habitability of some of these regions.